In recent years, there has extensively been developed an electronic system for generating a plate from electronic data stored in an appropriate data recording device, in the form of separation of a single color separated electronically, and there have been developed to be put in practice various processes to form electronically both a proof image and an actual plate, to make them to be stored and to process. Though some of these electronic systems can handle analog images, there are used digital images because they can be processed easily. Prior to complete judgment to decide whether final printing can be started or not, it is usually necessary to make a hard copy. Therefore, there is required a method to employ an output apparatus or a printer of a certain type which forms a hard copy or a proof image for the purpose of estimating finish of an actual printing.
Incidentally, Japanese Patent Publication Open to Public Inspection (JP O.P.I.) No. 5-221067 discloses an apparatus for forming proof images having a high image quality constantly, rapidly and accurately by means of a thermal transfer process In this thermal imaging apparatus, there are provided a roll medium supply device and a supporting member on which an image-receiving sheet and an ink sheet are superposed to be held thereon, and ink sheets can be removed without unexpected moving an image-receiving sheet on the supporting member even when various types of ink sheets are superposed on a single image-receiving sheet, then positions of images superposed plural times are confirmed while a part of the image-receiving sheet is held during an entire period of writing, and thus, final proofs are generated. On the surface of the supporting member stated above, there are provided openings for a vacuum so that the image-receiving sheet and the ink sheet are superposed and stuck to the drum-shaped supporting member, and there is further provided a vacuum sheet holding means which reduces pressure through the openings.
The thermal imaging apparatus stated above can constantly form proof images rapidly, accurately and at correct positions. However, large-sized proof images are demanded, and it is necessary to make the drum-shaped supporting member large when forming large-sized images on the apparatus as one stated above. When the supporting member is made large, it is difficult to superpose the image-receiving sheet and the ink sheet to hold them. The reason for the difficulty is as follows. When rotating a large-sized drum-shaped supporting member for exposure, a sheet tends to come off the supporting member. To avoid this, it is necessary to raise the degree of vacuum, which requires an apparatus which has a large capacity and is capable of generating the high degree of vacuum. In that case, air tends to leak through a joint of the drum-shaped supporting member, and an apparatus which can overcome these problems tends to be expensive and requires periodical maintenance. In addition, when winding a sheet round the supporting member, creases tend to be caused. Further, it is very difficult to attain the state of uniform close contact between the image-receiving sheet and the ink sheet over the entire surface thereof. When the state of uniform close contact can not be attained, there are caused serious problems on the finished proof images, such as a decline of stability of image output, namely, occurrence of a density difference in an image plane, irregular density in repeated output, occurrence of partial density fall and partial density rise in an image plane, and occurrence of blurred halftone dots.
Further, in the thermal imaging apparatus stated above, description is made on sheets of a type wherein dyes in an ink layer of the ink sheet are transferred. However, it has been made known that the same problems as in the foregoing are caused also on sheets of a thermal fusion transfer type wherein a colorant and a binder in the ink layer are transferred.